A micromachined ZnO/Si3N4 Lamb wave device for ultraviloet sensing application

Acoustoelectric (AE) effect, which arises from the interaction of acoustic waves and mobile carriers, is one of important sensing mechanisms of acoustic wave sensors. Nevertheless, investigations about the influences of AE effect on Lamb wave sensors in the literatures remain little thus far. In this regard, a micormachined 1.5 mm ZnO/Si₃N₄ Lamb wave device employing the AE effect for ultraviolet (UV) sensing application is studied both theoretically and experimentally. By introducing dispersion relations, a model associated with the acoustoelectric effect is modified to deal with the interactions of Lamb waves and mobile carriers. As numerical examples, Lamb wave propagation associated with the AE effect in ZnO/Si₃N₄/Si and ZnO/Si₃N₄ layered structures are discussed. Numerical results show attenuation of a ZnO/Si₃N₄ membrane due to the AE effect can be much more obvious than that of a ZnO/Si₃N₄/Si layered plate. On the experimental side, two types of Lamb wave devices, based on a ZnO/Si₃N₄/Si plate and an ultra-thin ZnO/Si₃N₄ membrane respectively, were realized and discussed. Under a 0.06 mWcm^-2 370 nm-UV illumination, a 1.8 dB insertion loss (IL) drop on the 1.5 mm ZnO/Si₃N₄ membrane and a 0.8 dB drop on the ZnO/Si₃N₄/Si layered plate were observed respectively. The experimental findings indicate that through proper designs, a micromachined ZnO/Si₃N₄ Lamb wave device can be a promising candidate for sensing applications using the acoustoelectric effect.